1. Field of the Invention
The present invention relates to a heat-sensitive recording material. More specifically, it relates to a heat-sensitive recording material having excellent properties such as high color-forming sensitivity (excellent thermal response), a high color density, freedom from ground fogging and excellent image retainability.
2. Related Art Statement
Heat-sensitive recording materials generally have a substrate and a heat-sensitive recording layer formed thereon, and the heat-sensitive recording layer contains, as main components, an electron-donating colorless or light-color dye precursor and an electron-accepting color developer. When a heat-sensitive recording material is heated with a hot head, a hot pen or a laser beam, the dye precursor and the color developer immediately react with each other to give a recorded image. Advantageously, the above heat-sensitive recording materials permit recording with a relatively simple-structured device, and such a device is easy to maintain and makes almost no noise, so that the heat-sensitive recording materials are applied to broad fields including measurement meters, facsimile machines, printers, computer terminals, labeling machines, ticket vending machines, and the like.
In recent years, heat-sensitive recording materials have come to be used as financial recording sheets for bills and receipts of gas, water, electricity, etc., slips of automatic teller machines and various receipts.
As the use and demands of heat-sensitive recording materials have expanded into various fields as described above, the heat-sensitive recording materials have come to be required to have a variety of properties such as high color sensitivity and a high color density, freedom from ground fogging, stabilized image retainability and excellent thermal response.
Specifically, a heat-sensitive recording material gives an image when heated. When a high color-forming sensitivity (excellent thermal response) and a high color density are realized in a heat-sensitive recording material, a ground (non-recorded portion) of the heat-sensitive recording material forms a color when exposed to a high temperature. This is a so-called ground fogging phenomenon. When the ground fogging is intense, there is caused a defect that the contrast of the ground to a recorded image disappears when the image-recorded article is exposed to a high temperature.
Further, there is another problem that an image-recorded article is degraded by chemicals contained in cosmetics and stationery articles, the infiltration of a plasticizer contained in wrapping films such as a vinyl chloride film or light such as sunlight and fluorescence lamp light. It is therefore desired to develop a heat-sensitive recording material which satisfies contradictory properties, such as high color-forming sensitivity (excellent thermal response) and saturation concentration in combination with the prevention of ground fogging, and which has excellent image retainability.
As means for improving the retainability to an image portion, there is proposed a heat-sensitive recording material containing, as an electron-accepting compound, a salicylic acid derivative having a substituent such as an alkyl, aralkyl, alkyloxy or acyl group or a metal salt thereof (JP-A-62-169681, JP-A-63-22683 and JP-A-63-95977).
However, the above heat-sensitive recording material containing the salicylic acid derivative is not sufficient for image retainability. Further, it is poor in thermal response, so that it cannot be said to be competent for practically fast recording.
For improving the thermal response, a sensitizer is added as required. When the sensitizer is melted itself under transmitted heat energy, it works to melt or include a dye precursor and a color developer in its vicinity to promote a color forming reaction. It is therefore one means of increasing the sensitivity of the heat-sensitive recording material to improve the sensitizer in thermal response and compatibility with a dye precursor and a color developer.
As the above means, attempts have been made to add, for example, waxes, a nitrogen-containing compound, carboxylates, a naphthol derivative, a naphthoic acid derivative, a benzoate derivative, p-benzylbiphenyl or diphenoxyethane. However, a heat-sensitive recording material obtained is not yet satisfactory concerning color density and thermal response.
JP-A-9-8605 and JP-A-10-35109 disclose a thermal paper which shows excellent recorded image retainability under a high-temperature environment at 80° C. or higher, causes ground fogging to a less extent and has excellent recording sensitivity.
The above thermal paper exhibits excellent recorded image retainability against heat at a temperature around the boiling point of water. However, when it comes into contact with a heater device having a temperature higher than the above temperature, a ground portion completely forms a color, and a difference between the optical density of a recorded portion and the optical density of a ground portion disappears. As a result, a recording no longer exists. Therefore, there have been practically obtained no thermal paper having satisfactory recorded image retainability against heat for securities and documents which have financial values in themselves.
Examples of the above contact to high-temperature heat source include a case where the simple lamination of a polyolefine resin is applied to a heat-sensitive recording material and a case where an electric iron is erroneously pressed to a heat-sensitive recording material. In the laminating, a heat-sensitive recording material is brought into contact with a heat source having a temperature of approximately 120° C., and in the latter ironing case, it comes in contact with a high-temperature heater having a temperature of approximately 150° C.
Meanwhile, the heat-sensitive recording layer of a heat-sensitive recording material which forms a color under heat to record an image contains a color-forming substance and a color-developing substance which reacts with the color-forming substance to allow it to form a color under heat. The color-forming substance is selected, for example, from colorless or light-color leuco dyes having a lactone, lactam or spiropyran ring, and the color-developing substance is selected, for example, from various acidic substances such as 4,4′-isopropylidenediphenol, 4,4′-dihydroxydiphenylsulfone, 2,4′-dihydroxydiphenylsulfone, 4-hydroxy-4′-isopropoxydiphenylsulfone and 3,3′-diallyl-4,4′-dihydroxydiphenylsulfone.
Since color-formed images obtained from a combination of the above color-forming substance and the above color-developing substance have a clear color tone, the above combination has been applied to various heat-sensitive recording materials. Of the above color-developing substances, 4,4′-dihydroxydiphenylsulfone is advantageous in that it has a high whiteness and a formed color is clear, while it has defects that it has a very high melting point (246° C.) and that a heat-sensitive recording material is therefore poor in thermal response so that the so-called sensitivity thereof is low. The heat-sensitive recording material has inferior color formability when used with fast facsimile machines and various printers to which recent energy-saving techniques are applied. The heat-sensitive recording material is therefore limited in use, and its improvement is strongly desired.
For improving the color forming performances, it is general practice to add a so-called sensitizer such as 2-benzyloxynaphthalene which works as a melting point dropping agent for the color-developing substance. As the above sensitizer, 2-benzyloxynaphthalene, hydroxydiphenylsulfone derivatives and benzyl oxalate derivatives are studied in various ways, while no sufficient effects are obtained at present.